Leadframes are commonly used as carriers for mounting IC chips during the assembly and packaging of semiconductor devices. The base material of the leadframe is typically made from copper or copper alloy. In the assembly process, an IC chip is mounted onto the leadframe used as a support structure, and electrical connections are made between the chip and the leadframe, typically by the welding conductive wires between respective contact pads on the IC chip and the leadframe. These connections allow electrical connectivity between the IC chip and external devices to which the semiconductor device may later be attached. Thereafter, the IC chip and wires are molded with an encapsulant, such as epoxy molding compound (“EMC”), to protect the IC chip and wires from the external environment.
In order to ensure proper functioning of the package, it is essential that the encapsulant adheres adequately to the leadframe. If such adhesion is not strong enough, the encapsulant may delineates from the leadframe, especially when the package encounters thermal stress at high temperatures and trapped water vapor evaporates, resulting in device failure or the so-called “popcorn failure”. Previously, with lead-based solder material, solder reflow temperatures to which the packages were exposed were lower, at about 220° C. However, with lead-free solders introduced due to environmental concerns, higher temperatures of up to 260° C. are required during soldering, thereby increasing the thermal stress inside the package and aggravating the risk of package failure.
One approach to enhancing encapsulant adhesion is to make the leadframe surface rough so as to improve the bonding strength between the leadframe surface and the encapsulant.
A prior art method of roughening the leadframe surface is described in U.S. Pat. No. 4,946,518 entitled “Method for Improving the Adhesion of a Plastic Encapsulant to Copper Containing Leadframes”. The adhesion of plastic encapsulants to leadframes containing substantial amounts of copper is improved by exposing the leadframes to an active oxygen ambient such as hydrogen peroxide at temperatures below the leadframe annealing temperature. The treatment strengthens any native oxide present on the leadframes, which is found to promote encapsulant adhesion. At the same time, exposed nickel on the leadframe used for bonding wires to the leadframe is cleaned by the active oxygen ambient.
However, whilst the disclosure is especially suitable for leadframes comprising nickel plating to facilitate wire bonding, nowadays the majority of IC packages use leadframes with sliver plating instead of nickel for this purpose. The presence of impurities formed as a result of the oxidizing process may impede the bonding between conductive wires and contact pads on the leadframe. Thus, the process is not satisfactory as it is not catered towards the cleansing of leadframes comprising plating other than nickel plating for the contact pads, such that impurities or contaminants may remain on the contact pads.
Another prior art process is disclosed in U.S. Pat. No. 7,049,683 entitled “Semiconductor Package including Organo-Metallic Coating Formed on Surface of Leadframe Roughened using Chemical Etchant to Prevent Separation Between Leadframe and Molding Compound”. In this process, the metal leadframe is specially treated by roughening it with a chemical etchant. In one embodiment, a leadframe made of copper is roughened with a chemical etchant that contains sulfuric acid and hydrogen peroxide. As a result, an organo-metallic coating is formed on the leadframe surface to reduce the possibility of separation between a molding compound and the leadframe as the package undergoes thermal cycles and/or to inhibit the ingress of moisture into the package.
In general, since specific portions of the copper leadframe are electroplated with metals such as silver, nickel or palladium to facilitate wire bonding between the leads of the leadframe and gold or copper wire during the assembly process, the interface between the said metallic plating on the leadframe and the wire material should ideally be free from any contaminant so as to promote good bond strength between the two materials. On the other hand, after the roughening process, the presence of an organo-metallic complex or copper ions left on the plating surface may degrade the bond strength between the leadframe and the bonding wire.
Accordingly, it would be beneficial to be able to remove impurities that are produced on the silver plated surface of the leadframes after conducting the roughening process. It would also be advantageous to develop a process that avoids the coating of organo-metallic material on the surface of the leadframe which may result in the degrading of the bond strength between the leadframe and the bonding wire.